This invention relates to apparatus intended for use in subsurface or other inaccessible locations, and in particular but not exclusively to such an apparatus having the ability to convert energy in fluid flowing in a subsurface conduit to energy which may be utilised by the apparatus. One aspect of the invention relates to a tractor.
In the oil and gas exploration and extraction industry, there is widespread use of subsurface and subsea conduits, often of extended length: for example, well bores may extend for several kilometers from wellheads to remote hydrocarbon-bearing formations, and subsea pipelines may extend for many kilometers between, for example, subsea wellheads and support platforms, and between production platforms and onshore facilities. Over time, there tends to be a build up of material on the inner walls of such pipelines, which material may include wax, scale and various precipitates. Of course such a build up of material will reduce the flow capacity of the pipeline, and may ultimately result in the pipeline becoming blocked. Conventionally, such pipelines are periodically cleaned by passing a xe2x80x9cpigxe2x80x9d through the pipeline, the pig being launched into the pipeline at an appropriate upstream point, which may require production to be temporarily stopped. A pig is typically of cylindrical form, of slightly smaller diameter than the pipeline, and is carried through the pipeline by the flow of fluid in the pipeline. Dislodged material therefore builds up in front of the pig, and thus it is not unusual for pigs to become stuck in pipelines, which may bring production to a halt. Retrieval of a stuck pig requires the pig to be located, the pipeline opened to remove the pig, and the pipeline reinstated. As these operations may have to be carried out subsea, retrieval of a stuck pig is an expensive and difficult undertaking.
It is among the objectives of embodiments of the present invention to obviate or mitigate these disadvantages. In particular, embodiments of the present invention relate to a tractor which may convert energy in fluid flowing in a pipeline to energy which may be utilised to drive the tractor in the opposite direction to the direction of fluid flow; the tractor may be utilised to transport a pipeline cleaning tool or device through a pipeline.
According to one aspect of the invention, there is provided a tractor adapted for travel through a moving fluid stream, the tractor comprising
a body;
drive means for moving the body in a desired direction;
a rotatable member mounted on the body and adapted to be driven by moving fluid; and
conversion means for converting movement of the member to drive for the drive means, said conversion means comprising a contactless coupling between the rotatable member and the drive means.
Such an arrangement enables the tractor to move in a moving fluid stream, and to obtain its motive power from the moving fluid. In preferred embodiments of the invention, this allows the tractor to operate without requiring a link to a remote power source, and without the tractor having to carry its own power supply. The tractor may be arranged to be driven in the same direction as the fluid, but is preferably arranged to be driven in the opposite direction to the fluid flow, that is upstream or against the flow.
Preferably, the member is adapted to be rotated by the flow of fluid past the body, and may be in the form of a turbine blade arrangement. The rotatable member preferably rotates around the body, and the axis of rotation of the rotatable member is preferably coaxial with the body.
Preferably, the conversion means is adjustable to vary the coupling between the rotatable member and the drive means. In one embodiment, the degree of coupling is variable and, for example, the maximum torque or force transferred between the rotatable member and the drive means may be varied, and in another embodiment the gearing between the rotatable member and the drive means may be varied. In other embodiments the degree of coupling between the rotatable member and drive means may be fixed, for example the conversion means may comprise a non-adjustable gear train.
Preferably, the conversion means comprises a magnetic induction coupling. This facilitates sealing the various elements of the tractor against ingress of fluid and other materials which may be present in the environment of the tractor, and may reduce friction losses normally incurred by sliding seals. In the preferred magnetic induction coupling, the spacing between the driving and driven elements of the coupling is variable, to vary the degree of coupling between the elements. Most preferably, the elements are relatively axially movable. Thus, by varying the axial spacing and thus the degree of coupling between the elements it may be possible to, for example, reduce the power or torque transferred from the rotatable member to the drive means and thus reduce the speed of the tractor. Alternatively, the conversion means may comprise a mechanical drive link, although for most applications this would require provision of sliding seals and the like.
The tractor may carry cutting means for removing or dislodging material surrounding or adjacent the body, for example deposits on the inner wall of a conduit through which the tractor is travelling. The cutting means may comprise a blade having radially or axially extending blades, and the blades may be rigidly mounted or in the form of flails. Alternatively, the cutting means may be in the form of fluid jets, or an ultrasonic emitter. In such embodiments, energy from the fluid driven member may be utilised to drive a pump or power an ultrasonic source.
The conversion means may be linked to a coupling control, which may control the degree of coupling between driving and driven elements of the coupling. The coupling control may comprise a rotating blade, and on the blade speed slowing, indicative of the tractor encountering a restriction or obstruction and the blade having to make a xe2x80x9cheavyxe2x80x9d cut, the degree of coupling may be reduced, to slow the rate of advance of the tractor. In other embodiments the coupling control may include a gauge sensor, such as a gauge ring.
Preferably, the conversion means comprises reduction gearing for converting a high speed low torque input from the rotatable member to a relatively low speed high torque output to the drive means. For example, the rotatable member may rotate at 2000 rpm, and this rotation may be stepped down at a ratio of 80:1, to 25 rpm. Preferably also, the conversion means comprises an output shaft which is substantially coaxial with an axis of rotation of the rotatable member. Most preferably, the conversion means comprises a harmonic drive. A typical harmonic drive comprises three main parts, (1) an outer rigid ring with internal teeth, (2) a non-rigid, thin cylindrical cup with external teeth on a slightly smaller pitch diameter than the outer ring, and the cup fitting over and being held in an elliptical shape by (3) an elliptical plug carrying a thin raced ball bearing. The cup is slightly smaller in diameter than the outer ring and usually had two fewer teeth than the ring. The elliptical shape of the plug causes the teeth of the cup to engage the ring at two opposite regions across the major axis of the ellipse. As the plug rotates, the zone where the teeth of the ring engage those of the cup travels with the major elliptical axis. For each 180xc2x0 clockwise movement of the plug, the cup moves counterclockwise by one tooth relative to the outer ring. Thus, each complete clockwise rotation of the plug results in the cup moving counterclockwise by two teeth from its previous position relative to the outer ring.
In other embodiments, the conversion means may comprise an electric generator, such as a dynamo or alternator, and an electric motor coupled to the drive means. This arrangement offers the advantage that the torque output of the motor is relatively constant, irrespective of motor speed. There is also some flexibility in the relative locations of the generator and motor, as there is no requirement for a mechanical coupling therebetween. The conversion means may include a battery or other electrical energy storage arrangement, the battery being supplied with electrical energy by the generator. An advantage of this arrangement is that the battery may provide motive power for the tractor in the temporary absence of fluid flow.
The conversion means may comprise a hydraulic fluid pump and a hydraulic motor, or may comprise one or more of a combination of energy conversion and transmission arrangements, to suit the particular application of the tractor.
The drive means may take any suitable form, and may include wheels, legs, tracks, fluid impellers or the like. Most preferably, the tractor is adapted for use in conduits, and the drive means is in the form of a plurality of brush traction elements, such as described in U.S. Pat. No. 6,460,616 B1, the disclosure of which is incorporated herein by reference. The brush traction elements are oscillated or swashed backwards and forwards by cams mounted on a central drive shaft and derive traction from elastomer or otherwise resilient fingers which contact the inner wall of the conduit, which may be a pipe or a cased or uncased borehole. Altering the mounting position of the fingers with respect to the centre line of the bearings on which each brush traction element is mounted increases or decreases the amount of grip provided by the fingers by increasing or reducing the effective diameter of each traction element. A cyclic variation in the mounting position of a series of adjacent traction elements, and which allows the traction elements to drive the tractor, may be achieved by varying the angular offset of each traction element cam on the central drive shaft. For example, each cam may be mounted in the drive shaft to be 60xc2x0, 90xc2x0, 120xc2x0 and 180xc2x0 offset from an adjacent cam. Further, by moving the mounting position of the brush traction element from one side of a neutral centre line to the other it is possible to reverse the direction of traction. This is useful when it is desired to retrieve the tractor; by moving the mounting position the tractor may then be driven in the reverse direction, or where the tractor has been moving against the direction of flow the force produced on the tractor by the fluid may be sufficient to push or reverse the tractor. To facilitate such movement the tractor may further comprise a selectively actuatable fluid flow restriction adapted to interact with the flowing fluid to create a force on the tractor. The restriction may be in the form of a parachute or like arrangement, or radially extendable seals or the like.
The tractor may comprise a plurality of bodies, each provided with drive means, a member, and conversion means. Alternatively, individual tractor elements may be provided selectively on the bodies. The bodies may be coupled by any appropriate means, for example flexible cables, chains or universal joints, and where drive or other forces are to be transferred between bodies by, for example, constant velocity universal joints. The ability to form the tractor from a number of flexibly coupled or articulated bodies facilitates negotiation of tight bends, including 3D and 5D bends (that is tubes having a bend radius three or five time the tube radius) as often utilised in oil and gas exploration and production applications.
Preferably, the tractor includes an obstruction sensor operatively associated with one or both of the conversion means and the drive means and for reversing or uncoupling said means to permit the tractor to stop or move in an opposite direction. The sensor may comprise a probe at the leading end of the tractor for detecting solid obstructions, or detecting sharp turns in a tube in which the tractor is located; for example, the probe may be adapted to detect a 3D bend in a tube, that is a bend with a radius of three times the radius of the tube. Thus, the tractor may be adapted to travel through a tube until the tractor encounters a 3D bend at a known location, and on encountering the bend the tractor will reverse. Alternatively, the sensor may comprise a gauge ring or cutting blade.
The body preferably defines a housing accommodating the conversion means, and the housing is preferably pressure compensated.
According to another aspect of the invention there is provided a tractor adapted for travel through a moving fluid stream, the tractor comprising:
a body;
drive means for moving the body in a desired direction;
a rotatable member mounted on the body and adapted to be driven by moving fluid; and
conversion means for converting rotation of the member to drive for the drive means, said conversion means comprising a harmonic drive.
In other aspects of the invention, one or both of a harmonic drive and a contactless coupling may be provided in apparatus other than a tractor for converting or transferring movement of a fluid actuated member to another element of a tool or device, such as an electrical generator, a cutting blade or a pump.